There are a few reasons the code is still there:

1. The `platform_img()` code would have to also update the `data-src` attribute so that `initHighResImages()` would append to the correct value. Alternatively, `initHighResImages()` could grab the `src` attribute and append `high-res` to that.
2. The execution order of `$(document).ready()` events would always have to be `global.js` followed by `mozilla-high-res.js` or else we'd be appending the the `high-res` suffix before the platform suffix. This could probably be enforced though.
3. If we changed the code to one of the patterns I describe in point 1, we'd be setting the `img.src` property twice, which would probably cause two HTTP requests (one for the normal-res and one for the high-res).